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Red blood cell distribution width
Measure of red blood cell volume variation as part of a standard blood test

Red blood cell distribution width (RDW) measures the variation in red blood cell size and is reported in a standard complete blood count. While normal red blood cells range from 80–100 femtoliters, disorders like anemia cause increased size variation, reflected in a higher RDW-CV, normally between 11.5–15.4%. RDW, combined with mean corpuscular volume, helps differentiate anemia causes. Deficiencies in Vitamin B12 or folate lead to macrocytic anemia with elevated RDW in many cases, while iron deficiency anemia almost always shows increased RDW. This variation, called anisocytosis, isn't present in all anemias, such as anemia of chronic disease or hereditary spherocytosis, which can have normal RDW values.

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Calculations

The "width" in RDW is sometimes thought to be "misleading", since it in fact is a measure of deviation of the volume of RBCs, and not directly the diameter. RDW-CV "width" refers to the width of the volume curve (distribution width), not the width of the cells. 78

RDW-SD is calculated as the width (in fL) of the RBC size distribution histogram at the 20% height level. This parameter is, therefore, not influenced by the average RBC size (mean corpuscular volume, MCV).9

RDW-CV (expressed in %) is calculated with the following formula:

RDW-CV = (1 standard deviation of RBC volume ÷ MCV) × 100%.10

Since RDW-CV is mathematically derived from MCV, it is therefore affected by the average RBC size (MCV).11

Pathological implications

Normal RDW

Anemia in the presence of a normal RDW may suggest thalassemia. A low Mentzer Index, calculated from CBC data [MCV/RBC < 13], may suggest this disorder but a hemoglobin electrophoresis would be diagnostic. Anemia of chronic diseases show normal RDW.

High RDW

High RDW may be a result of the presence of fragments, groups of agglutination, and/or abnormal shape of red blood cells.12

  • Iron-deficiency anemia usually presents with high RDW and low MCV.
  • Folate and vitamin B12 deficiency anemia usually presents with high RDW and high MCV.
  • Mixed-deficiency (iron + B12 or folate) anemia usually presents with high RDW and variable MCV.
  • Recent hemorrhages typically present with high RDW and normal MCV.
  • A false high RDW reading can occur if EDTA anticoagulated blood is used instead of citrated blood. See Pseudothrombocytopenia.

By severity, elevated RDW can be classified as follows:

RDW in adults13Severity14Most notable causes15
14.5% - 18%Mild anisocytosisInfection, vigorous exercise or certain drugs. Iron deficiency anemia in the presence of anemia and microcytosis.
18% - 26%Moderate anisocytosisSevere iron deficiency anemia, megaloblastic anemia.
> 26%Severe anisocytosisSideroblastic anemia

Further reading

References

  1. Nah EH, Kim S, Cho S, Cho HI (November 2018). "Complete Blood Count Reference Intervals and Patterns of Changes Across Pediatric, Adult, and Geriatric Ages in Korea". Annals of Laboratory Medicine. 38 (6): 503–511. doi:10.3343/alm.2018.38.6.503. PMC 6056383. PMID 30027692. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6056383 ↩

  2. Nah EH, Kim S, Cho S, Cho HI (November 2018). "Complete Blood Count Reference Intervals and Patterns of Changes Across Pediatric, Adult, and Geriatric Ages in Korea". Annals of Laboratory Medicine. 38 (6): 503–511. doi:10.3343/alm.2018.38.6.503. PMC 6056383. PMID 30027692. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6056383 ↩

  3. "Red Cell Distribution Width". Family Practice Notebook. Retrieved 18 October 2013. http://www.fpnotebook.com/hemeonc/lab/RdClDstrbtnWdth.htm ↩

  4. Nah EH, Kim S, Cho S, Cho HI (November 2018). "Complete Blood Count Reference Intervals and Patterns of Changes Across Pediatric, Adult, and Geriatric Ages in Korea". Annals of Laboratory Medicine. 38 (6): 503–511. doi:10.3343/alm.2018.38.6.503. PMC 6056383. PMID 30027692. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6056383 ↩

  5. Evans TC, Jehle D (1991). "The red blood cell distribution width". The Journal of Emergency Medicine. 9 (Suppl 1): 71–4. doi:10.1016/0736-4679(91)90592-4. PMID 1955687. /wiki/Doi_(identifier) ↩

  6. Kjeldsberg CR, Perkins SL (2010). "Table 1.4". Practical Diagnosis of Hematologic Disorders (5th ed.). Chicago, IL: ASCP Press. ISBN 978-0-89189-571-8. Archived from the original on 25 July 2013. 978-0-89189-571-8 ↩

  7. Pomeranian Medical University > Red blood cell indices (indexes)[permanent dead link‍] Retrieved on 5 April 2009 http://www.ams.edu.pl/files/File/wronog/Metodyki-unit_4c.pdf ↩

  8. Cornell University, Department of Population Medicine & Diagnostic Sciences > Red Cell Distribution Width (RDW) Last Updated: June 2000. Retrieved on 5 April 2009 http://diaglab.vet.cornell.edu/clinpath/modules/hemogram/rdw.htm ↩

  9. Curry, Choladda Vejabhuti (7 January 2017). "Red Cell Distribution Width (RDW): Reference Range, Interpretation, Collection and Panels". Medscape Reference. Retrieved 18 May 2019. https://emedicine.medscape.com/article/2098635-overview ↩

  10. Encyclopedia of Surgery: A Guide for Patients and Caregivers Red blood cell indices Retrieved on 5 April 2009 http://www.surgeryencyclopedia.com/Pa-St/Red-Blood-Cell-Indices.html ↩

  11. Curry, Choladda Vejabhuti (7 January 2017). "Red Cell Distribution Width (RDW): Reference Range, Interpretation, Collection and Panels". Medscape Reference. Retrieved 18 May 2019. https://emedicine.medscape.com/article/2098635-overview ↩

  12. Curry, Choladda Vejabhuti (7 January 2017). "Red Cell Distribution Width (RDW): Reference Range, Interpretation, Collection and Panels". Medscape Reference. Retrieved 18 May 2019. https://emedicine.medscape.com/article/2098635-overview ↩

  13. "High RDW level in the blood". MrLabTest. Last update: 12/01/2021 https://www.mrlabtest.com/high-RDW-blood.htm ↩

  14. "High RDW level in the blood". MrLabTest. Last update: 12/01/2021 https://www.mrlabtest.com/high-RDW-blood.htm ↩

  15. "High RDW level in the blood". MrLabTest. Last update: 12/01/2021 https://www.mrlabtest.com/high-RDW-blood.htm ↩